Since high-temperature exhaust gas exhausted from an engine passes through members in an automobile exhaust system such as an exhaust manifold, a front pipe, and a center pipe, a material configuring the members in an exhaust system is required to have a variety of characteristics such as oxidation resistance, high-temperature strength, and thermal fatigue characteristics.
In the past, it was usual to use cast iron for members in an automobile exhaust system; however, from the viewpoint of the intensification of exhaust gas regulations, the improvement of engine performance, a decrease in the weight of a vehicle frame and the like, there has been a trend of using more stainless steel exhaust manifolds. The temperature of exhaust gas varies depending on the types of vehicles, and the temperature of exhaust gas has been frequently in a range of approximately 750° C. to 850° C. in recent years, but there have also been cases in which the temperature of exhaust gas reaches a higher temperature. In an environment in which members in an exhaust system are used in the above-described temperature range for a long period of time, there is a demand for a material having high high-temperature strength and high oxidation resistance.
Among many types of stainless steel, austenitic stainless steel has excellent thermal resistance and workability. However, since austenitic stainless steel has a large thermal expansion coefficient, thermal fatigue failure is likely to occur in the case where austenitic stainless steel is applied to a member that is repetitively heated and cooled such as an exhaust manifold.
Compared with austenitic stainless steel, ferritic stainless steel has a lower thermal expansion coefficient; and therefore, ferritic stainless steel has excellent thermal fatigue characteristics. In addition, compared with austenitic stainless steel, ferritic stainless steel rarely contains expensive Ni; and therefore, the material cost is low, and thus ferritic stainless steel is widely used. However, ferritic stainless steel has a lower high-temperature strength compared with austenitic stainless steel; and therefore, techniques that improve high-temperature strength have been developed.
Examples of the above-described techniques include SUS430J1L (Nb-added steel), Nb—Si-added steel, and SUS444 (Nb—Mo-added steel) in which the high-temperature strength was improved by adding Si and Mo in addition to the basic addition of Nb. Among the above-described techniques, SUS444 had the highest strength since approximately 2% of Mo was added, but there were problems in that the workability was poor and the cost was high due to its high content of expensive Mo.
Therefore, in addition to the above-described alloys, a variety of additive elements have been studied. Patent Documents 1 to 4 disclose Cu addition techniques in which the solid solution strengthening of Cu and the precipitation strengthening of Cu using a precipitate (ε-Cu phase) are used.
However, there is a problem in that the addition of Cu degrades oxidation resistance. Oxidation resistance denotes two points that the mass gain is small without causing abnormal oxidation and the resistance against scale spallation is favorable.
In the case where stainless steel is heated, highly protective scales mainly containing Cr2O3 are generated in the surface. Cr is required to maintain the highly protective scales, and when Cr is not sufficiently supplied from a base metal, Fe is oxidized. At this time, in an oxide containing a large amount of Fe that is generated, the oxidation rate is extremely large. Therefore, the oxidation proceeds rapidly, and the base metal is greatly eroded. The above-described phenomenon is called abnormal oxidation.
In Patent Document 5, causes for the degradation of oxidation resistance by the addition of Cu are assumed. Cu is an austenite-forming element, and due to a decrease in the amount of Cr in a surface layer portion in response to the progress of oxidation, the phase transformation from the ferrite phase to the austenite phase is promoted only in the surface layer portion. Since Cr diffuses slowly in the austenite phase compared with the ferrite phase, when the austenite phase is formed in the surface layer portion, the supply of Cr from a base metal to scales is hindered. Then, it is assumed that the surface layer portion becomes deficient in Cr, and the oxidation resistance deteriorates. Therefore, a technique is disclosed which improves oxidation resistance by mutually adjusting a ferrite-forming element and an austenite-forming element and suppressing the austenite phase based on what has been described above.
However, even when favorable scales not causing abnormal oxidation can be formed, it is a problem if the scales are spalled off in, for example, the cooling process of an automobile exhaust system or the like. When the scales are spalled off, oxygen in the atmosphere comes into contact with the base metal during heating, and oxidation proceeds rapidly. If the scales cannot be protectively reproduced, abnormal oxidation may be caused. In addition, when the spalled scales are scattered, there is a possibility of the occurrence of problems such as the erosion of devices on the downstream side or the blocking of flow channels by the accumulation of the scales.
The scale spallation in members in an automobile exhaust system is frequently caused in the case where the thermal expansion difference is great between the base metal and an oxide and in the case where heating and cooling are repetitively carried out, and thermal stress is considered as a principal cause for the scale spallation. Since a thermal expansion difference between ferritic stainless steel and scales is smaller than a thermal expansion difference between austenitic stainless steel and scales, ferritic stainless steel is superior in terms of the resistance against scale spallation. In addition, a variety of techniques that improve the resistance against scale spallation in ferritic ferritic stainless steel have been developed.
Patent Document 6 discloses a method in which the Mn/Si ratio is adjusted to form a large amount of a Mn-containing spinel-based oxide having an intermediate thermal expansion rate between the thermal expansion rates of an oxide mainly containing Cr2O3 and the base metal; and thereby, the adhesion of scales is improved. However, it is necessary to set the Si concentration to be extremely higher (0.80% to 1.20% by mass %) than the Si concentration in ordinary ferritic stainless steel, and there is possibility of the workability being impaired. In addition, there is no disclosure regarding the thickness of the scales and the relationship between the shape of the interface between the scale and the base metal and the resistance against scale spallation.
Patent Document 7 discloses a method in which a small amount of Al is added to make scales fixed by “growing roots”, but it is necessary to set the Si concentration to be extremely higher (0.80% to 1.50% by mass %) than the Si concentration in ordinary ferritic stainless steel, and there is possibility of the workability being impaired. In addition, there is no disclosure regarding the relationship between the thickness of the scales and the resistance against scale spallation.
Patent Document 8 discloses a method in which the integrated content of Mo and Si is regulated since the adhesion of Cr2O3 oxide and Si oxide is poor; however, the Si content is in a range of 0.10 wt % or less which is extremely lower than the Si content in ordinary ferritic stainless steel. In the case where Al is used as a deoxidizing agent, it is difficult to set the Si content to be in a range of 0.10% or less, and there is a possibility of the cost increasing. In the case where Al is not used, when the Si content is 0.10%, there is a concern of poor deoxidation, it becomes difficult to decrease the S content to an extremely low level, and there is a possibility of the cost increasing. In addition, there is no disclosure regarding the thickness of the scales and the relationship between the shape of the interface between the scale and the base metal and the resistance against scale spallation.
Patent Document 9 discloses a method in which the interface between scales and the base metal is made to be greatly uneven and entangled together and Ti is added to strengthen the scale-fixing action. However, since the Ti concentration is in a range of 0.23% to 1.0% by mass % which is extremely higher than that of ordinary ferritic stainless steel, there is a possibility of uniform elongation, hole expansibility, toughness and the like being impaired. In addition, there is no disclosure regarding the relationship between the thickness of the scales and the resistance against scale spallation.
According to what has been described above, the knowledge of the related art to improve the resistance against scale spallation of members in an automobile exhaust system was mainly about the improvement of the resistance against scale spallation by controlling the scale composition using Mn, Si, and Mo and the improvement of the resistance against scale spallation by controlling the shape of the interface between the scales and the base metal using Al and Ti, and there was no disclosure of knowledge to improve the resistance against scale spallation by controlling the thickness of scales. In addition, there was no disclosure of knowledge to improve the resistance against scale spallation by controlling the shape of the interface between the scale and the base metal using Mn and Si. Furthermore, it is necessary to control the Si content or the Ti content to be extremely high or low; and with the contents, there is a possibility of impairing workability, cost, uniform elongation, hole expansibility, toughness and the like. Therefore, there was no technique to improve the resistance against scale spallation in the Si or Ti content range of ordinary ferritic stainless steel.
In addition, while the reason is not clear, the addition of Cu degrades the resistance against scale spallation. In Patent Documents 6 and 7, the Cu content is in a range of 0.80% or less, and there is no assumption regarding the degradation of the resistance against scale spallation. That is, it was necessary to develop techniques to improve the resistance against scale spallation in Cu-added steel.
As described above, Cu-added steel is desirable for members in an automobile exhaust system in terms of high-temperature strength and cost, but there is a problem with oxidation resistance, particularly, the resistance against scale spallation.